Question

why can 3+4 be written as 3 + 3 + 1

Answer 1

because 3+3+1= 7 which is the same number 3+4 is equal to. Another reason is any number can be broken down into smaller numbers that equal the whole. 

Answer 2

the reason why 3+4 can be written as 3+3+1 because they both will have the same answer 3+4=7 and 3+3+1=7 and even if u switch the numbers 3+3+1 an d switch 3+4 they will still have the same answer like this  
                  4+3=7 and 3+1+3=7